Project Summary Children?s early externalizing behavior problems, including symptoms of Attention- Deficit/Hyperactivity Disorder (ADHD) such as inattention, hyperactivity, and impulsivity, are the most common reason for early childhood mental health referrals and occur in 10- 25% of preschoolers. Despite the successful development of evidence-based treatments for ADHD, early interventions have been shown to have little impact on children?s long- term academic and social impairment. A major barrier to understanding the long-term treatment impact on children with ADHD is that current definitions of the disorder rely solely on DSM-V symptom profiles. A simplistic classification system limits our understanding of the heterogeneity present in ADHD, particularly during the preschool and early elementary school years. The heterogeneity of the disorder suggests that the ADHD may be characterized by multiple subgroups with varying neuropsychological profiles, with the assumption is that these profiles reflect different underlying neurobiological substrates. However, an established understanding of the neurobiology of ADHD is lacking, in particular, in the executive function (EF) and emotion regulation (ER) domains. In accordance with the NIMH strategic plan (Strategy 1.4) we seek to overcome these limitations by identifying distinct ?biosignatures? derived from an integration of neurobiological (functional and structural magnetic resonance imaging (MRI); functional MRI and diffusion-weighted MRI) and pathophysiological markers of EF and ER. This will aid in the early identification and tailored treatment of behavioral and neuropsychological phenotypes of ADHD. Additionally, we propose to examine the extent to which the identified ?biosignatures? predict children?s early intervention response. Understanding and better capturing the heterogeneity of EF and ER may lead to more targeted treatments to improve children with ADHD?s social and academic functioning.